How do I add a semi-transparent color over an image in gimp?

Example of what I'm trying to do is the radar maps in MechWarrior 3: you have a "satellite view" of a landscape and the whole image has a semi-transparent green hue and you can see all the details of the landscape. It's as if it were done in black and white, but it's different shades of green.

What I am trying to do: the radar maps from Pirate's Moon, MechWarrior 3's expansion pack, are not colorized this way, and I wish to duplicate the idea above on those maps, and I am using Gimp... any ideas how I can do this?